Diving With Gas Mixes Other Than Air

 

Larry "Harris" Taylor, Ph.D.

This is an electronic reprint of my material that appeared in the history chapter of MIXED GAS DIVING published by Watersport.  This material is copyrighted and all rights retained by the author. This article is made available as a service to the diving community by the author and may be distributed for any non-commercial or Not-For-Profit use.

It was called "A Study In Controlled Paranoia." It was the deepest ever dive made by a free-swimming scuba diver.  On April 5, 1988, Sheck Exley added another exploration to his personal log. This event was a milestone in diving history; it was not a typical sport dive. Sheck plunged to a depth of 780 feet within the Mexican cave system known as Nachimento Del Rio Mante. 

The descent took 24 minutes; the decompression time was longer than 10 hours. The decompression times and gas mix utilized were controlled by a computer generated protocol developed by Bill Hamilton and Dave Kenyon known as the DECAP (Decompression Computation and Analysis Program). Since there was no previous experience with free-swimming "sport" divers at 700 feet, the tables were considered experimental. In addition to the four cylinders that Sheck carried, the dive protocol called for 16 staged bottles at depths between 30 and 270 feet, containing 11 different gas mixes, with 52 separate decompression stops beginning at 520 feet. The dive ended at the surface with breathing pure O2 for 30 minutes. 

The dive was the essence of the true explorer: a highly skilled individual testing theoretical decompression protocols while venturing into areas previously unknown to mankind. If this had been a pilot's adventure, Hollywood would already have released the movie (starring Errol Flynn), and Sheck would be battling Teenage Mutant Ninja Turtles for prime time ratings. As it was, after finishing the dive, Sheck and his team loaded the van and drove home... just another day of diving. 

The cave diving community has consistently been on the cutting edge of technology. Their continual drive to go deeper and further than anyone before has given rise to much of what is known in the recreational dive community about the use of gas mixes other than air for deeper diving. Sheck's dive was not an isolated event, but rather the pinnacle of a series of carefully planned advances in knowledge about deeper diving.  The advancement of knowledge has not been without its price. Along the way, pioneers and adventurers have died while seeking to dive deeper, extend bottom time and cover more distance.  We, who sport dive today, owe our "recreating" to those who have gone before.

HELIUM

In 1919, Professor Elihu Thompson, an electronics engineer and inventor, speculated that nitrogen narcosis could be avoided if the oxygen in the breathing mix were diluted with a gas other than nitrogen. Thompson had previously established a record as an innovator with seven hundred patents including electric welding, the centrifugal cream separator and street arc lights. His business had earlier merged with Thomas Edison's company to form General Electric. He suggested that helium would be a suitable gas for deep diving without narcosis.  Since at the time, the price of helium was over $2500 per cubic foot, the suggestion was viewed as an economic impossibility. About this time C.J. Cooke applied for a patent on the use of Helium as a breathing gas mix. Additionally a series of experimental dives was begun on the U.S.S. Falcon, which included at least one dive to 150 feet on a heliox mix. Later, the discovery of helium in four Texas natural gas wells gave the United States an exclusive monopoly on the world's supply of helium. Its abundance dropped the price of helium to a few cents per cubic foot. 

Thompson convinced the Bureau of Mines, which controlled the world's supply of helium (and was desperately seeking some use for this gas), and the US Navy to begin examining the potential for deep diving using helium and oxygen as a breathing gas mix. By 1925 a lab had been established in Pittsburgh and lab animals were doing simulated dives in a chamber using helium-oxygen mixes.  This work established that animals breathing an 80 % helium / 20% oxygen mix could be decompressed at 1/6 the decompression time of an air breathing animal.  Later, humans subjects breathing 80% helium / 20% oxygen were found to have no apparent problems with heliox decompression schedules that were 1/4 the time required for air breathing dives. More importantly, however, was the ability for humans to function "clear-headed" at depths where air-breathing divers were incapacitated by nitrogen narcosis. Thompson's second major contribution to deep diving followed this early effort. He suggested that since the helium was not consumed during the dive, it could be conserved by use of a re-circulating system for the diluent gas. His idea would ultimately prove sound; it needed only the development of high efficiency absorbents to remove the carbon dioxide generated from human metabolism so that the exhaled gas could be re-circulated. 

Divers commented on the ease of breathing helium, but noticed that they always felt chilled while breathing heliox mixes. The change in voice characteristics often made communication at depth difficult. It was apparent that the narcosis free advantage of helium breathing would create problems as well as solutions. It was obvious that much work still needed to be done. 

Funding for deep diving training was very difficult to obtain in the post World War I economy. Although the US Navy Experimental Diving School in Newport, Rhode Island had successfully trained divers that had salvaged the sunken submarine F-4 in 304 feet of water off Amala Bay near Honolulu, Hawaii in 1915, Congress could not be convinced to provide the funds for continuing the US Navy Experimental Diving School in Newport.  This school had, before the First World War, conducted over three hundred test dives on air to depths of 258 feet. This work was the foundation for the first diving manual published in 1924 by the Bureau of Construction and Repair. 

On September 25, 1925, the submarine S-51 sank after a collision with the S.S. City of Rome near Block Island in 132 feet of water.  Since the Navy had not had the funds to maintain their deep diving training program and associated submarine rescue proficiency, the salvage of this vessel took many months. (There simply were not enough divers qualified to dive below 90 feet. Bad weather and extreme cold also hampered the salvage effort.) Despite the disaster and modest public inquiry, funding was still denied. However, the public began to slowly build interest in improving the Navy's ability to function at deeper depths. 

In 1927 another sub, the S-4 sank in 102 feet of water with loss of all forty men. One sad aspect of this disaster is that six men survived the sinking and their taps on the hull could be heard by divers working to raise the vessel. For a short time, two-way communication via rapping on the hull existed until, eventually, the taps from inside the hull ceased.  The salvage operation lasted for over three months.  An indignant public began to demand an improved Navy capability. As a result, the Navy established an Experimental Diving School in Washington, D.C. The two primary missions were to develop diving techniques to limit the effects of nitrogen narcosis and to develop rapid, effective methods for the rescue of crew trapped inside sunken submarines. 

One significant early achievement of this Navy research unit was the development of the McCann rescue bell. This was a diving bell or chamber that could be lowered on cable to mate with the escape hatch on a submarine. Once attached, the water in the chamber between the bell and the submarine could be blown away by the use of compressed air. This then allowed trapped submariners to open their hatch and move into the rescue bell. The bell hatch could be closed and the chamber pulled back to the surface by winch. This bell would prove itself as this rescue device saved many trapped submariners. 

In the late 1930's, an intern at Milwaukee County General Hospital, Edgar End, investigated the use of helium and oxygen as a breathing gas mix. His friend, Max Gene Nohl, an MIT graduate student, had developed with the assistance of John Craig (later of "Danger Is My Business" television fame), a new type of diving helmet.  This helmet was part of a self-contained helium-oxygen system with on-board scrubbing of carbon dioxide that had been developed with the goal of photographing the Lusitania in 312 feet of water. During their pre-expedition trials, they were able to work at 312 feet for up to two hours with only three brief stops on their thirty minutes ascent to the surface.  When the expedition to photograph the Lusitania was abandoned, it was decided to attempt a world depth record. On a cold December day in 1937 the self-contained heliox helmet was tested in near-by Lake Michigan. Max Nohl set the world's depth record at 420 feet. The dive was conducted using a suit that was definitely not conventional.  The diver wore a fashionable rubber coverall with Eskimo-like mukluks boots and a helmet that was described as looking like a lighthouse top with windows on all sides. The diver wore two self-contained tanks of breathing gas. 

About the same time, the US Navy research program began gathering significant momentum under the guidance of Behneke and Yarbrough.  They successfully completed a simulated chamber dive to 500 feet using heliox. It is interesting to note that during the Navy tank dive, the diver did not know his depth. When asked about his depth, the diver replied, "It feels like a hundred feet." During his decompression, the diver was told his actual maximum depth.

This success prompted the Navy to increase investigative efforts in the use of helium in diving gas mixes. By 1939 experimental research was a reality when the submarine, U.S.S. Squalus, sank off the Isles of Shoals in 243 feet of cold North Atlantic water.  Since the submarine had been quickly located, there was a frantic effort to rescue the men trapped on board. A downhaul cable (for the McCann Rescue Bell) had parted and a diver on compressed air had been unable, due to the crippling effects of nitrogen narcosis, to repair the cable. It was decided to try the new helium-oxygen mixture.  A diver on heliox was successful.  The McCann rescue bell made four trips in twelve hours to the sunken submarine and 33 men were successfully rescued. The submarine was then salvaged. The US Navy conducted over 100 dives without injury in the rescue/salvage effort. The U.S.S. Squalus, renamed the Sailfish, served in World War II.  The rescue of thirty-three men and the successful salvage of the Squalus demonstrated to the US Navy and the American public that heliox was a viable protocol for deep diving operations. The successful rescue of these trapped men and the subsequent salvage of the sunken vessel are considered to be two of the most significant accomplishments in the history of marine life saving and salvage operations. The completion of the Squalus salvage and the appearance of war clouds on the horizon prompted Congress to increase the funding for US Navy deep diving training and research.  Incidentally, Congress, fearing both the ingenuity of German research under the political control of Adolph Hitler and the use of helium in lighter-than-air dirigibles, prohibited the export of helium.  For the next twenty years the US Navy, with the world's sole supply of helium, was the primary user/investigator of heliox as a breathing gas mix. 

Jack Browne, devised a triangular lightweight mask and tested his system in 1946. Diving in a pressurized tank, with decompression guidance from End and Behnke, he did a simulated dive of 550 feet. 

Following the Second World War, the British, using helium that had been obtained with the approval of the US Secretary of Interior, Secretary of Defense and the US President, began experimenting with heliox mixtures. In some of their first dives, the divers developed extreme claustrophobia during diving and in screaming fits demanded to be hauled out of the water. Although the physiologists were convinced that the oxygen concentrations were too high and thus the source of the problems, the divers blamed the helium. It was referred to as "Yankee gas" or "Stuka juice." (The Stuka was a German dive-bomber used during WW II.) 

The British diving experiments were conducted from the vessel Reclaim. This vessel anchored near a vertical wall in Loch Fyne that bottomed out at 540 feet. Since the Olympics were being held in London, Captain Shelford devised a "diver's depth thermometer" and sent divers on eighteen dives, each deeper than the previous. The depth's reached were recorded on the "thermometer." A Diver's torch was created and each new depth reached was rewarded with the diver receiving the "Olympic torch." The diver would accept the torch and run a victory lap around the vessel while wearing diving boots.  Using nearly all of the remaining British supply of helium, Diver First Class Wilfred Bollard took 7 1/2 minutes to descend to a new world record depth of 540 feet. After 5 minutes on the bottom the diver took 8 hours and twenty-six minutes to ascend (descent time was increased because of a three hour treatment for the bends; the diver suffered elbow pain when transferring from the Davis submersible decompression chamber (which he had entered at 190 feet) to the main decompression chamber at 30 feet.)  After leaving the decompression chamber he received the "torch" from the "hand" of a Neufeldt and Kuhnke armored diving suit. Since this state-of-the-art suit was rated to 500 feet, Bollard scratched "OUT OF DATE" on the suit and then did his victory lap!   

In 1968 Heliox entered the world of "amateur" cave diving. A group of BSAC divers primarily from Rhodesia and South Africa, spent more than year assembling gear, support equipment and personnel to dive the Silent Pool in Sinoia, Rhodesia. Divers Roly Nyman, lan Robertson, John van der Walt and Danny van der Walt dove to 340 fff on Tri-Mix (Nitrogen  41.6%: Helium  40 %: 18.4% Oxygen) while exploring the muddy bottom of this famous sink-hole.

In 1970, ex-Navy diver Tom Mount brought heliox into American scientific diving at the University of Miami Rosenteil School of Marine Science. About the same time, Hal Watts, in Florida,  experimented with heliox for deep cave operations.

Dale Sweet, in 1980, used heliox to reach 360 feet in the cave Die Polder #2. Although six months later, Sheck Exeley made the same dive on compressed air, the cave diving community was beginning to notice the existence of mixed gases for deep explorations. It would take three years, but in 1981, a German cave diver, Jochen Hasenmayer, descended into the French Vaucluse to reach a depth of 476 feet using heliox.  This was a new world record for a surface-to-surface scuba dive. In 1983, Jochen made another heliox dive. This dive was to 685 feet and another world record. 

One of the most well known uses of mixed gas within the cave diving community was the 1987 exploration of the Wakulla Springs Cave system. Using twelve divers, the team penetrated the cave more than 4000 feet at depths near 300 feet. The dive operation decompression profiles were controlled by a new computer analysis protocol developed by Bill Hamilton and Dave Kenyon. Much of the success of the exploration was due to the use of mixed gas under the new decompression guidelines. Now, with reasonable decompression predictions, the depth records would move downward and more cave systems would be systematically explored. 

Sheck Exley, a living cave diving legend, personally took up the challenge to extend the depth range of cave divers. After practice heliox dives in Florida springs, Sheck went to Mexico and made "tune-up" cave dives of 515 and 660 feet. Later, On April 1, 1988, he extended the world record for free swimming scuba divers to 780 feet. 

HYDROGEN

The first recorded use of hydrogen as a breathing mix was in 1789. Lavoisier (The Father of Modern Chemistry) and Sequin exposed guinea pigs to mixtures of hydrogen and oxygen (Hydrox). They observed that the animal's oxygen consumption appeared to be similar in hydrogen/oxygen as in nitrogen/oxygen. Prior to WW II, a Russian scientist, Lazarev, subjected a single mouse to elevated pressures of hydrogen and oxygen. However, the use of hydrogen as a breathing gas for diving operations is generally associated with the Swedish Engineer, Arne Zetterstrom. Hydrogen is a desirable breathing gas component because it is the lightest element known. This means it is the least dense at depth and breathing resistance is minimal at extreme pressures. The major problem with hydrogen-oxygen mixtures is the potential for explosion. Although the concentration of oxygen needed for combustion of oxygen-hydrogen mixes varies a bit with pressure, a general rule of thumb is that hydrogen-oxygen mixes above 5 %  O2 are at-risk.. So, to avoid nasty fires and explosions, hydrogen is only considered as a breathing gas component at pressures where a less-than 5% oxygen concentration in the breathing gas mix gives a partial pressure of oxygen great enough to sustain life. 

Perhaps the most common example of a hydrogen-fire related disaster is the destruction of the Hindenburg dirigible. The nearly 900 foot long airship burned on landing near Lakehurst, NJ  in the late 1930's after a successful trans-Atlantic crossing. While there is no doubt that the hydrogen burned and its flames consumed the dirigible, the most likely cause of the fire was not a hydrogen-oxygen problem.  Recent analysis suggests a slightly more different scenario. The airship was coated with a metal-particle coating (that's how it got its classic silver sheen.) It is now believed that this material acquired a substantial electrical charge because of near-by thunder storm activity. The landing officials on-site requested, because of the weather, that the vessel drop its landing lines from a high altitude. It is believed that in-doing so, as soon as the lines hit the ground, the vessel "grounded-out" and a substantial electrical charge was passed. The electrical current carried by the metallic particles in the skin coating created enough sparks to ignite the hydrogen and once ignited, the hydrogen burned with enough intensity to destroy the once greatest-ever airship. This incident will always remind us of the potential for disaster that is present whenever oxygen and hydrogen are used in proximity..

In 1944 Arne Zetterstrom discovered a way to breach the transition between compressed air and Hydrox without risking explosion. The technique was to descend to 100 feet and switch to a 4% oxygen / 96 % nitrogen mixture. After breathing this mix for sufficient time to allow the oxygen concentration in the lungs to drop below the "explosion threshold," the diver switched to Hydrox and continued descent. On ascent, the diver again used the Nitrox (4% O2 / 96% N2) as a transition between Hydrox and air. Using this technique, he descended to 363 feet. At that depth, the alteration in voice characteristics, coupled with excitement, made communication impossible and additional dives used a telegraph key. 

On August 7, 1945 Arne intended to demonstrate the usefulness of his technique for assisting in submarine rescue efforts by establishing a new world record. It is believed that he dove on a support platform suspended from the stern of the vessel. Due to intense current, a line was rigged from the bow of the vessel to the support platform to maintain vertical orientation. He dropped rapidly to 100 feet on compressed air and made the Nitrox transition to Hydrox. Then, tapping telegraph messages during the entire descent, he dropped to a record setting 528 feet. Since no Hydrox tables existed at the time, Arne used his own calculations for the ascent. His first staged decompression stop was intended to be 165 feet. The bow winch stopped at 165, but an enthusiastic, but misguided stern crew continued raising him to the surface. There was no gas transition and he ascended while breathing 4% oxygen/hydrogen with no decompression stops. He died shortly after reaching the surface. The death certificate stated that death was from "acute lack of oxygen and caissons disease of a violent nature." Although his death was totally unrelated to the use of hydrogen or his transition technique, (but by what was termed "an unpardonable mistake"), research on this gas was discontinued for many years. 

During the mid 1960's research into the use of hydrogen in breathing gases resumed with animals breathing Hydrox for up to 24 hours at 70 Ata. One interesting aspect of the animal research was the suggestion that hydrogen reduced the HPNS (high pressure nervous syndrome) often observed with helium based gas mixes on deep dives. Ultimately animals would be taken to 3500 feet on hydrox. 

By 1967 there were two successful human chamber dives using hydrogen as a breathing mix to 7 Ata for 10 and then 20 minutes. More experimentation was resumed during the early 1970's to begin to develop realistic tissue saturation times for the future development of Hydrox decompression tables .In 1974 the US Navy initiated a series of dives termed HYDROX to further evaluate problems associated with switching from Hydrox to other gases. 

In 1983 COMEX, the French deep diving concern (perhaps more famous in the US as the company providing the submersible used in the recovery of artifacts from the Titanic) began a series of dives to investigate the narcotic potential of hydrogen. Divers including H.G. DeLauze, President of COMEX, descended in open sea to approximately 300 feet for five minutes. The divers could not perceive a difference between Hydox and Heliox at that depth.  Chamber dives to 300 m (984 ft) demonstrated that hydrogen possessed a narcotic effect different from nitrogen. Hydrogen narcosis (the "hydrogen effect") had a tendency to be more psychotropic � i.e. more like LSD, while nitrogen narcosis had an effect similar to alcohol. This deeper work suggested that Hydrox as a binary gas mix would not be too useful at depths below about 500 feet. 

MIXMASTERING 

A young mathematician and Zurich engineering school instructor, Hannes Keller, saw his first aqualung while vacationing in Greece in 1958. After talking with local divers and with no previous experience in diving he was said to have proclaimed, "Diving techniques were thirty years behind the times." He decided that his life's work would be to improve diving technology. A voracious reader, he soon had read much of what was then available on deep diving technology. He convinced a cardiac specialist, Albert Buhlmann to join in his efforts. Together they developed a technique for utilizing nine different gas mixes at various depths and proposed that deep diving was possible by varying the proportions of the gases at different depths. 

Although no money was available for computer time, Keller convinced IBM to give him four hours of computer time to do the necessary calculations for developing tables based on the hypothesis of multiple mixes for different depths. The result was four hundred different tables for depths up to 1312 feet. Next, he went to Jacques Cousteau and with his assistance was able to have access to the French diving chamber that was operated by the French Navy Group for Undersea Research and Development at Toulon. There, Cousteau and a team of Swiss scientists watched Keller attain a pressure of 630 feet and ascend without illness or apparent difficulty. When news of his success circulated within the international diving fraternity, this community assumed that Keller was some sort of physiological freak with extraordinary tolerances for breathing exotic gas mixes at depth. 

Keller went to the US to try to convince the US Navy to finance further work. Although many in the US scientific diving community were intrigued by Keller's claims, funding was denied. (Keller insisted that he would retain all commercial rights and that his gas mixtures remained a secret.) Keller then returned to France. 

Although the French could not fund his work, they did allow him access to the chamber in Toulon. Here Keller did a simulated dive to 1000 feet using a commercially available sport diving Calypso single hose regulator. This got the world's attention, but unfortunately helped promulgate the myth that Keller was indeed some freak of nature that could survive events that would kill mere mortals. 

In an effort to convince the world of the validity of his diving theories, in 1961 he established a diving barge on Lake Maggiore near the Swiss-Italian border.  Here, with Kenneth MacLeish (a 40 year old Science editor for Life magazine with some deep diving scuba experience) as a dive buddy, he attempted to prove that his success was due to the nature of his theory, not some unusual physiology.  Wearing rubber suits and woolen underwear, the pair descended on an elevator to 725 feet of fresh water (equivalent to 685 feet of sea water). After inscribing their initials on a mermaid doll that they had brought with them for luck, they took one hour to ascend to the surface. Apart from the cold, they suffered no apparent ill effects with no symptoms of the bends. 

This event convinced many that Keller's work should be taken seriously. In 1962, Keller with the financial support of the US Navy, some publishers and an oil company, descended with Peter Small (a journalist and founder of the British Sub Aqua club) in a chamber designed by Keller, The Atlantis, to a depth of 1000 feet off Catalina Island. On the bottom Keller left the chamber and planted Swiss and American flags. The flags were very large and Keller became entangled in them.  After struggling to get free, he returned to the bell with only moments remaining in his gas supply.  In his desperate reentry at depth, Keller's fin caught in the gasket that sealed the chamber. Without a gas tight seal, breathing gas was lost. (The chamber had been designed to maintain a pressure inside greater than outside. A fin compromised the integrity of the seal and thus resultant seal loss meant that gas would rapidly escape.) Surface observers watched the two men on television as they lost consciousness.  After the chamber had reached free diving depth (200 feet), Richard Anderson and Chris Whittaker, dove to the chamber and freed the fin.  Only Anderson returned.  On return to the surface Small was found dead.  Small died of an embolism; the cause was attributed to lack of oxygen while he was unconscious. An inquest chaired by John Craig found death to be accidental; the result of exploration of the unknown coupled with human stress to complete the dive.  The board concluded, "Keller's experiment produced a significant scientific achievement." While the experiment ended in tragedy, it did demonstrate the viability of using multiple gas mixes at depth. 

The trend has been to investigate and use multiple component mixes for extended diving. It was found in 1965 that divers breathing heliox mixtures at depths below 500 feet developed nervous tremors known as High Pressure Nervous Sydrome (HPNS). A series of deep dives at the F.G. Hall laboratory at Duke University under the direction of Peter Bennett found that using small quantities of nitrogen in the heliox (termed tri-mix) helped eliminate this problem. In their Atlantis dive series, three divers reached a depth of 2132 feet breathing tri-mix. 

It has been found that adding helium to hydrogen-oxygen mixes (termed Hydreliox) helps to eliminate the "hydrogen effect" narcosis associated with breathing only a hydrogen-oxygen mix. Theoretical limits of hydreliox are currently placed at about 1750 feet. 

NITROGEN

Henry Fleuss, Master Diver for Siebe, Gorman & Co. of London, conducted the first documented dive using oxygen-rich air. Breathing an estimated 50-60% O2, Mr Fleuss spent an hour in a large tank. A week later, he used his apparatus in open water. He was injured when tenders on the dive abruptly pulled him to the surface. The dives were conducted in 1879. By 1912 Robert Davis and Leonard Hill had devised a self-contained rigid diving helmet that utilized a 50% oxygen-nitrogen mixture. This apparatus was used under the guidance of J.S. Haldane to a working limit of 100 fsw. Since the nitrogen concentration was much less than air, this device demonstrated the then-remarkable decompression advantage available from using an oxygen-enriched air breathing mixture. 

The first commercial use of nitrogen-oxygen mixes of other than normal air concentration was the self-contained dress of the Westfalia Machinenfabrik in Geisenkirchen, Germany.  In 1912 they used their suit with a mixture of 45% and 55% O2 for depths to a 100 feet and a 30% O2 mix for diving to depths of 200 feet. This suit, or the Nitrox blend did not receive wide distribution. Based on this work, in 1913 Draegerwerk produced a similar device that automatically mixed nitrogen and oxygen supplies to produce a 60 percent O2 mix. Some time before WW I, the Fleuss-Davis SCUBA unit appeared. This device consisted of two 10 cubic foot tanks; one each for compressed air oxygen. The gases were mixed in a manifold between the two tanks and the diver's mouthpiece. The manufacturer claimed success of this unit to depths of 66 feet. 

Between the two world wars Siebe Gorman & Co. introduced the technique of using different concentrations of oxygen mixed with nitrogen. It had been established that divers could not tolerate oxygen concentrations greater than 2 Ata for extended periods of time without difficulty. The divers affected by this incapacity and convulsions associated with high O2 concentrations invented a mythical monster, "Oxygen Pete," who was supposed to lurk on the bottom of the sea waiting to molest unwary divers. Oxygen toxicity hits during this time were referred to as "getting a Pete."  

Perhaps the best-kept secret of WWII was the use of oxygen-enriched air re-breathers by the British commandos defending Gibraltar. Those attacking the British strong-hold were using 100% O2  re-breathers.  The deeper maximum operating depth of the British mixes (45-60% O2) was a distinctive underwater combat advantage since opposing divers (using 100 % O2) would be at �convulsive depths� while the British divers were still within their operating parameters. A major component of the British strategy was to simply take the opponent down until convulsions overwhelmed the enemy diver. This secret was so well kept, that much of this was not even revealed to the US Navy until the 1950�s. 

One interesting feature of the British combat protocols was the definite association between CO2  build-up and increased susceptibility to oxygen toxicity seizures. Their orders forbade rapid swimming unless �demanded by enemy contact.�  Much of our present knowledge of oxygen enriched air mixes can be traced directly to the British research efforts on oxygen and oxygen-enriched air breathing mixes conducted during WWII. 

Since it was known that increased oxygen (decreased nitrogen) increased time available bottom time without decompression obligation in the 60 -100 foot range, a number of mixes were utilized primarily by the commercial diving community during the period following WW II. 

Workman developed decompression schedules for nitrogen-oxygen and helium-oxygen diving and published these tables in 1965. 

In the late seventies, the Canadian research institute DCIEM was asked to develop a diving apparatus for the Canadian military to be used in clearing mines. The ideal system would not disturb mine sensors that would detect motion, magnetic fields, and/or sound. It was decided to utilize a semi-closed system that would use a nitrogen-oxygen mix that would vary in O2 concentration at depth to supply a constant p O2. The constant pO2 is delivered via a pneumatic manifold, as opposed to an electronic pO2 sensor controlled relay system. This system was made available in the late 80's. 

In 1978 NOAA formally established procedures for a standard mixture of 68% N2 / 32% O2. It is known as NOAA Nitrox I. A second standard mix containing 36% O2 is known as NOAA Nitrox II. Since that time, a number of users, including the US Navy, commercial and academic diving operations have successfully used Nitrox in operations shallower than 130 feet. NOAA has developed a reasonable compact shipboard continuous gas mixing system to supply Nitrox for diving operations. 

During the last five years, approximately 28,000 logged dives using Nitrox were surveyed. Although rigorous statistical analysis is not yet complete, the trend is that Nitrox is a safe, easily handled mix when used by properly trained divers. Two different agencies, ANDI and IAND, have been formed to introduce this technology to the sport diving communities. The sanctioning of Nitrox training by recreational training agencies NAUI and NASDS indicates that Nitrox mixes are becoming a permanent part of the sport diving community. 

FUTURE HISTORY 

"What improvement may here after be made in diving I will not pretend to say; yet I am convinced that there can be much progress in the art." Johnny Green wrote these words in 1859. They are still valid!